No incoming call sound when already talking on phone using headphones - how to change?

When I'm already talking on the phone through a headset I get no audible notification or vibration when another call comes in. I do get the ringtone played through the headphones when the first call comes in, and I do hear a beep when a second call comes in and I'm not wearing headphones.

I've looked hard through the settings, plus searched for prior questions that pertain to this and any apps that might help, all without any luck.

Phone is 1st gen Moto X running 4.4.4

I have not rooted the phone, but am not opposed to doing so if it would help, just haven't bothered with this phone yet.